The apparatus and method described in Pesavento and Strawbridge U.S. Pat. No. 4,085,685 teach the manner in which absorption spectroscopy can be successfully used in the testing of chemical solutions, and particularly in electroplating solutions. Koenigsberg and O'Neal describe, in U.S. Pat. No. 4,989,942, a useful structure for creating the light gap for positioning in the liquid solution. In this case, reliability is stressed by protection of the components against the adverse electroplating environment.
Different solutes may have spectroscopic peaks at different wavelengths. It is this characteristic which permits identification of a particular solute. However, these wavelengths may be best served by different light sources, and these different light sources may be best transmitted through different types of optical fibers. Furthermore, the peaks may be best detected at different detector gaps. As a result, for a sensor system to have a fairly wide utility, it is necessary to be able to adjust the light gap, as a function of concentration and transparency of the liquid stream, change light sources and/or filters to provide light in the proper wavelength band for which the solute is being, and to be able to change the optical fiber to one which is most suitable for the particular wavelength band. The sensitivity of the system is determined by its optical path length across the gap through which the stream passes. An increase in path length yields greater sensitivity: greater differentiation of lower concentrations. However, with a greater path length and corresponding increased light absorption, the amount of light reaching the detector may drop below the limit for accurate measurement. A variable path length allows the system to be adjusted for maximum sensitivity within the operating range of the detector. Thus, there is a need for providing a sensor and system with an adjustable, selectable path length. Fiber optic cables come in different types, each with its spectral window of maximum transmittance, depending on the material used in the optical fiber. If a sensor is to be used at different wavelengths, it is also necessary to change the fiber. Thus, changing the fiber optic cable to a different material is also necessary to provide wide utilization of the sensor. A sensor with interchangeable cables is not limited to any one cable and its corresponding spectral range, but such a sensor can use any fiber optic cable.